Method of producing cathodes for electronic tubes



g 29, 1950 G. GALLET EI'AL METHOD OF PRODUCING CATHODES FOR ELECTRONIC TUBES Filed April 9, 1947 FM. 1a

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Patented Aug. 29,.

METHOD OF PRODUCING CATHODES FOR ELECTRONIC TUBES Georges Gallet and Nguyen Thien-Chi, Paris,

France, assignors to Oompagnie Generale de Telegraphic Sans Fil, a corporation of France Application April 9, 1947, Serial No. 740,418v In France March 5, 1946 Section 1, Public Law 690, Aiigust s, 1946 Patent expires March 5, 1966 6 Claims. (Cl. 25027.5)

In its common form, an oxide cathode comprises a nickel tube the operative surface of which is covered with alkaline-earth oxides and within which there is located, without being in contact therewith, an insulated filament which, when brought to a red heat, heats the cathode by radiation. It is advantageous to reduce the power thus consumed to a minimum, by using it in the most efiicient manner.

I. The chemico-physical nature of the supporting metal has a considerable influence on the emission of the layer of oxides: it is therefore important for this metal to have very precise and very constant properties inorder to ensure satisfactory working of the cathode.

The application of the particular techniques of the metallurgy of powders to the construction of cathodes, which is the subject of the present invention, enables the requirements above mentioned to be fulfilled and-also provides other advantages which are no less appreciable.

According to the principles of the invention, the method essentially comprises giving to the cathode the shape of a pellet ofcompressed powder in which is embedded a heating filament covered with an insulating layer which is 'sufiiciently strong and adherent to withstand, without damage, on the one hand, the action of pressure applied during the moulding operation and, on the other hand, the effects of temperature firstly during the baking operation and subsequently during the normal operation of the valve; this notwithstandingthe difference in the ratio of expansion as between the metal of the filament and that of the pellet.

For this purpose, a first layer of powder on which the filament is arranged is first compressed lightly; a second dose of powder is then added, and the final pressure is applied.

On the other hand, a very simple means for centering the filament consists in moistening it with pure'alcohol, for example,*and sticking it in the correct position on the head of the punch: said punch is then gently inserted until it comes into contact with the first layer of powder on which it deposits the filament which is thus automatically centered in the matrix.

In order that the filament be equidistant from both faces of the pellet, it is, of course, necessary to proportion suitably the weights of the two layers of powder, according to the nature and the physical characteristics of said powder, and to adjust the preliminary and final pressures according to the size of the filament and the properties of the powder, in particular its fineness.

According to the invention, the compressed pellet is subjected to baking either in an atmosphere of hydrogen or in a vacuum. It remains quite fiat if it has been moulded under a sufficient pressure and if the baking is correctly effected. Moreover, any deformation of the pellet is prevented by holding it between two plates pressed together by screws.

Cathodes manufactured in accordance with the method of this invention may be called by the trade name monobloc cathodes; they have numerous advantages, especially that of enabling the heating power to be used with maximum efi'iciency, since there is intimate contact between the filament and the metal of the oathode yet ensuring absolute uniformity of temperature over the entire cathode surface. When the cathode is in operation, it exhibits an absolutely uniform red colour, which is an indication that there is no local overheating adjacent the filament, despite the close proximity of the latter.

By way of a non-limitative example, there will now be described, a very simple process of manufacture which avoids the need for having recourse to complicated moulds: it comprises giving to the pellet the shape or a racket, a plan view of which is shown in Figure 1, while Figure 1a shows a section on the line A-B' of Figure 1. The filament F may be of any shape, loop, hair-pin and so forth, with its two leads f1, f2 lodged in the handle of the racket P. It only remains to strip the said leads below the level of the line XY, in order to permit the current supply wires to be welded to them: in the case of nickel, which is the metal most frequently used, the metal of the racket is dissolved by attacking it with acid or, better still, by electrolysis. This operation may be effected either before or after baking.

Insulation is obtained by the use of substances which have an exceptionally high resistance at high temperature, such as, periclase, magnesite, and so forth, and which are deposited on the filament by cataphoresis under precise conditions of potential difierence, current intensity, concentration and fineness of grain. A very strong and very adherent coating is obtained by means of a special fritting operation conducted in a suitable atmosphere.

According to another techniqu in accordance with the invention, the cathode is formed by two twin half-pellets, enclosing the filament between them, said filament being lodged in a groove provided in each half-pellet. Figure 2 shows a plan view and Figure 2a a section of a half-pellet P with its groove E containing the filament F, th two ends of which protrude and are ready to be welded to the current input leads without'having to be stripped beforehand as in the previous case. This circumstance is especially to be appreciated in the case of a tungsten cathode, tungsten being a metal which is difficult to dissolve without attacking the filament at the same time. The groove is produced by means of a former embossed on the end of the punch. A series of half-pellets of the same weight are thus manufactured and it is then only necessary to assemble them in pairs. For this purpose, all that is required, in many cases, amongst which is the use of nickel, is to press the two halfpellets one against the other in a small press and to subject the whole assembly to baking. The; assembly of the two parts is brought about by the combined effects of the temperature and the pressure. In other cases, use is made of an intermediary substance in the form of a metallic paint.

According to another aspect of the invention, the monobloc process may advantageously be applied to metals which are hard to work mechanically'as, for example, thoriated tungsten: the method of the invention makes it possible to eliminate the very delicate operations of hammering, wiredrawing and rolling the metal.

The incorporation of added substances in the supporting metal of an oxide cathode exerts a substantial eifect on the electron emission of these oxides. In the case of nickel, the presence of aluminum, titanium, magnesium, considerably increases the emission. The use'of the metallurgy of powders enables such additions to be effected in an accurate manner, without having recourse to fusion during which the metal is often contaminated by the absorption of solid or gaseous impurities.

When the monobloc cathode contains certain metals, such as Mg, Ba, and the like, the said metals vaporize in a vacuum and condense on. the cold portions of the bulb in the form of a brilliant anduniform deposit. This. method Of manufacture can be used in making photo-electric cells, or again the said metals can act as getters.

Finally, it may be observed that the method according to the invention enables emitting cathodes, or emitting anodes in the, case of emissions of ions, of very varied natures to be obtained readily, since it is unnecessary to pass through a phase of the complete metallurgy relative, thereto, as is the case with nickel cathodes such as are used in thepresent-day technique.

We claim:

1. Methodof manufacturing indireetly heated emitting cathodescontaining a heating filament embedded in a coated pellet of metallic powder, which method comprises applying to a heating filament'an' appropriate thin insulating coating layer, inserting thecoated filament in a groove of suitable shape and dimensions provided in the inner surface of a half pellet of metallic powder, ap plying another half, pellet of the same shape, made 'of-"the same powder, by its inner surface, to the first half pellet to form a rigid block, and applying to said pellet on its outer surface a layer of thermo-emissive substance.

2. Indirectly heated emitting cathode comp1'is ing a heating filament, a thin insulating coating directly applied on the filament, a body of sintered metallic powder forming a rigid block in intiinatefcontact with the said insulated filament which is embedded therein, and a layer of thermo-emissive substance directly deposited on the surface of the said body.

3. Indirectly heated emitting cathode comprising a heating filament, a thin insulating coating directly applied on the filament, a body made of sintered metallic powder and forming a rigid block in intimate contact with the said insulated filament which is embedded therein, the thickness of said coating being substantiall less than the thickness of said body, and a thermo-emissive layer directly deposited on the surface of said body.

4. Cathode as claimed in claim 2 in which the said body of sintered. metallic powder is mixed with an mission. act vat ubstan e.-

5. Indirectly heated emitting cathode comprising a heating filament, a thin insulating coating directly applied on the filament, a, body of sin; tered metallic powder forming a rigid block, shaped, in the form ofia pellet, in intimate contact with the saidinsulated filament which is embedded therein, andthermo-emissive layer direct,-v ly deposited on the surface of, said pellet.

6. Indirectly heated emitting cathode comprise ing a heating filament, a thin insulating coating directly applied on the filament, a body of, sintered, metallic powder, forming a rigid block in intimate contact with thesaid insulated filament which is embedded therein, the, thickness of said coating being substantially less than the thickness of said body, the said block being shaped in the form of a racket, in the, handle ofwhich are. n i cdcied. he ads. f, the filament. and, he mqm sive a r. ir ctl dspp ted on. h e. of. t e. aid make;-

GE RGES. QUYEN. HiLEu-CH witssse s 1. 1 1).

The following references are of record in th file of this patent: Y

ED TAT S, BAT T TS 

2. INDIRECTLY HEATED EMITTING CATHODE COMPRISING A HEATING FILAMENT, A THIN INSULATING COATING DIRECTLY APPLIED ON THE FILAMENT, A BODY OF SINTERED METALLIC POWDER FORMING A RIGID BLOCK IN INTIMATE CONTACT WITH THE SAID INSULATED FILAMENT WHICH IS EMBEDDED THEREIN, AND A LAYER OF THERMO-EMISSIVE SUBSTANCE DIRECTLY DEPOSITED ON THE SURFACE OF THE SAID BODY. 